Super absorbent polymer (SAP) is a synthetic polymer material capable of absorbing moisture from about 500 to about 1,000 times its own weight, and each manufacturer has denominated it as different names such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material) or the like. Such super absorbent polymers started to be practically applied in sanitary products, and now they are widely used for preparation of various products, for example, hygiene products such as paper diapers for children or sanitary napkins, water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultice or the like.
In most cases, these super absorbent polymers have been widely used in the field of hygienic materials such as diapers or sanitary napkins. For these applications, the super absorbent polymer should exhibit a high moisture absorbency, it should not release the absorbed water even in the external pressure, and additionally it should well retain the shape even in a state where the volume is expanded (swelled) by absorbing water, and thereby exhibit excellent liquid permeability.
In particular, in recent years, as studies have been conducted to provide diapers exhibiting excellent performance while having a thinner thickness and a light weight, much attention has been focused on providing a super absorbent polymer having more improved absorption properties such as a liquid permeability and an absorption rate. In order to achieve excellent absorption properties such as a fast absorption rate and improved liquid permeability, it is necessary that the surface strength of the super absorbent polymer particles, particularly the surface cross-linked layer, is made harder and the gel strength is higher. Consequently, it becomes necessary for urine to be evenly and rapidly dispersed in the absorber core of the diaper.
However, in the case where it is desired to increase the gel strength and increase the liquid permeability through a method previously known in the art, there were a drawback that the basic absorption performance (absorbency under no pressure and under pressure) itself is greatly lowered.
Therefore, there is a continuing need to develop a technique capable of providing a super absorbent polymer having improved gel strength, liquid permeability, absorption rate and the like while maintaining excellent basic absorption performance.